/**
 * https://leetcode.cn/problems/validate-binary-search-tree/
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode() {}
 *     TreeNode(int val) { this.val = val; }
 *     TreeNode(int val, TreeNode left, TreeNode right) {
 *         this.val = val;
 *         this.left = left;
 *         this.right = right;
 *     }
 * }
 */
class Solution {
    public long prev = Long.MIN_VALUE;

    public boolean isValidBST(TreeNode root) {
        if(root == null) return true;

        boolean left = isValidBST(root.left);
        //剪枝
        if(left == false) return false;

        boolean cur = false;
        if(root.val > prev) cur = true;
        //剪枝
        if(cur == false) return false;

        prev = root.val;
        boolean right = isValidBST(root.right);
        return left && right && cur;
    }
}